This invention relates to chemical process equipment in which a liquid is contacted with a counterflow of gas. This may be for a variety of purposes such as stripping a component from the liquid stream or absorbing a component into a liquid stream. More generically this invention relates to equipment designed to facilitate mass and/or heat transfer between phases.
The type of equipment to which this invention specifically relates employs cross-flow fractionation trays connected by downcomers. In such equipment a tower is provided with a plurality of fractionation trays arranged generally horizontally within the tower. Each tower has a perforated deck and at least one channel, called a downcomer, in which a liquid flowing over the deck may be collected and channeled to the tray below. In use a gas or vapor is introduced at the base of the tower and passes upwards through the perforations in the decks of the fractionation trays. Meanwhile a liquid is introduced at the top of the tower and percolates downward passing over the fractionation trays and down the downcomers to the tray below. Liquid exits the downcomers in a typical design either through an open bottom and/or the downcomer front area, (that is the side facing towards the center of the tray). In some cases the downcomer may have a bottom pan where liquid flows around and out through slots in the bottom. In some designs there is provision for a perforated area under the downcomer with contact between vapor and liquid precluded by a box over the perforated portion of the under-downcomer area. Normally no provision is made for vapor contact with the liquid in the under-downcomer area and this reduces the capacity of the tower. Some modifications to address this problem have led to the use of slotted bottoms to the downcomers to distribute the liquid flow more evenly so as to allow perforations of the under-downcomer area but this is still a problem area from the point of view of liquid passing directly down through perforations intended for the upward passage of vapor.
According to the ideal process design, the liquid should be prevented from passing through the perforations in the decks of the fractionation trays by the pressure of gas passing through the perforations in the upward direction. This is a finely balanced process since, if the pressure is too great, the gas will have a shorter transit time within the tower and less efficient contact with the down-flowing liquid. The high gas velocity may also cause liquid droplets to be carried up to the tray above, thereby reducing the separation efficiency as a result of back-mixing. On the other hand if the gas flow rate is too low the liquid will penetrate through the perforations in the tray decks, (known as "weeping"), and short-circuit the flow patterns which are intended to maximize liquid/gas contacts.
Thus, in summary, the gas flow should be slow enough to permit efficient contact with the liquid flow but fast enough to minimize weeping. While a pressure differential between the space above a fractionation tray and the space below is necessary, if this differential is too great gas flow will be accelerated as it passes through the perforations and the efficient bubbling contact will be lost. To maintain the same volume of gas flow but reduce the pressure differential it is necessary to maximize the perforated area of the fractionation tray or provide some other efficient mechanism for the gas to contact the liquid as it passes through the fractionation tray.
Weeping is however often a problem when the liquid flow rate is particularly heavy in a local perforated area, and particularly in the under-downcomer area. Therefore to some extent the desire to attain the greatest contact efficiency, (which implies the lowest feasible pressure drop across the tower), by perforating as large a part of the deck surface as possible is at odds with the desire to avoid weeping. The present invention provides a high efficiency, (or high capacity), fractionation tray design that ensures that the danger of weeping is minimized.